Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for releasing a torque in a seismic cable section prior to inserting various sensors inside the seismic cable.
Discussion of the Background
Marine seismic data acquisition and processing generate a profile (image) of the geophysical structure (subsurface) under the seafloor. While this profile does not provide an accurate location for oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of reservoirs. Thus, providing a high-resolution image of the subsurface is an ongoing process for the exploration of natural resources, including, among others, oil and/or gas.
During a seismic gathering process, a seismic survey system 100, as shown in FIG. 1, includes a vessel 102 that tows plural seismic sensors 104 distributed along a seismic cable 106. Vessel 102 may tow plural seismic cables 106 at the same time. The seismic cables may be disposed horizontally, i.e., lying at a constant depth z1 relative to the ocean's surface 110. Also, the plural seismic cables 106 may form a constant angle (i.e., the seismic cables may be slanted) with respect to the surface of the ocean as disclosed in U.S. Pat. No. 4,992,992, the entire content of which is incorporated herein by reference.
Still with reference to FIG. 1, each seismic cable may have a head float 106a and a tail float 106b connected to its respective ends for maintaining given depth z1. A front-end gear 112 that includes various cables connects seismic cables 106 to vessel 102. Vessel 102 also tows a sound source 120 configured to generate an acoustic wave 122a, which propagates downward and penetrates the seafloor 124, eventually being reflected by a reflecting structure 126 (reflector). The reflected acoustic wave 122b propagates upward and is detected by seismic sensors 104. For simplicity, FIG. 1 shows only one path 122a corresponding to the acoustic wave.
The recorded traces may be used to determine an image of the subsurface (i.e., earth structure below surface 124). However, to produce a high-quality image of the subsurface, the seismic sensors used in the seismic cable need to perform as designed, i.e., to have actual characteristics that conform with the design characteristics envisioned by the design engineer. In addition, the seismic sensors are supposed to be aligned with one or more given directions within the seismic cable, and during the processing phase, this alignment is assumed to be obeyed by all seismic cables. If the alignment is not present, the recorded seismic data might be wrongly processed, generating inaccurate images of the surveyed surface.
More specifically, to eliminate much of the unwanted noise received by particle motion sensors in a seismic cable, it is desirable to form an array of sensors. The benefits of arrays are well-known in the art of acoustics, and the same benefits are achieved when particle motion sensors are combined to form an array. Sensor arrays provide a method to reduce unwanted noise by forming a spatial filter which can be tailored to receive the desired signal and attenuate unwanted noise.
To form an array or group of analog particle motion sensors, each sensor needs to be aligned relative to a given direction of the seismic cable so that individual sensors in the array receive signals from the same direction. This alignment normally takes place during the manufacturing phase, when the seismic cable has inherent torque. Note that inherent torque may be a consequence of manipulating, manufacturing, rolling, etc., of the cable during manufacturing or transport (e.g., from the manufacturing facility to the vessel designated to tow the seismic cable). When the seismic cable is towed through the water during normal data acquisition, there are no forces acting on it to counteract its inherent torque and, therefore, the seismic cable is free to rotate about its axis. When this cable rotation happens during data acquisition, it negatively impacts the recorded data because sections of the cable will have different orientations, resulting in sensors belonging to a same array having different orientations.
This problem faced by traditional manufacturing processes is illustrated in FIGS. 2A-C. FIG. 2A shows a seismic cable 200 considered to have, among many sections, sections 202 and 204. Sections 202 and 204 do not have to be physically different portions of the seismic cable. Sensors 202a in section 202 and sensors 204a in section 204 are aligned during the manufacturing process so that they all have their axes 210 and 220 parallel with, e.g., gravity, which is represented by axis Z in the figure. A cross-section view of seismic cable 200 is shown in FIG. 2B, illustrating that all the sensors' orientations coincide, i.e., arrows 210 and 220 are aligned with axis Z. This is during the manufacturing phase, when inherent torque is present in seismic cable 200. This is so because the seismic cable is supported during this phase by parts attached to a bench that cause considerable friction, which makes the release of inherent torque difficult, if not impossible. In other words, during manufacturing, frictional forces between the seismic cable and the bench do not allow torque to be relieved because the seismic cable is not free to rotate as it would in the water column, when towed by a vessel.
Thus, when the seismic cable is towed behind the vessel, frictional forces between the water and seismic cable are greatly reduced, and inherent torque has the opportunity to be relieved. During this process, section 204 may rotate relative to section 202 so that arrows 210 and 220 of corresponding sensors 202a and 204a are offset as illustrated in FIG. 2C, which is a cross-sectional view of seismic cable 200 during operation. Especially if sensors 202a and 202b are connected together to form an array, their different orientations in the same array introduce inaccuracies in the recorded data, which is undesirable.
Existing seismic cable technologies use either a single sensor for the group or MEMS devices to detect particle motion. The use of single sensors does not provide a method to attenuate inherent vibration in the seismic cable, which results in a highly noise-contaminated signal. MEMS devices can be used to form an array digitally, but digital signals from the MEMS devices must be individually processed. This requires additional data channels and, therefore, much higher data bandwidth and increased power consumption.
Thus, for those seismic cables using particle motion sensors combined in arrays, there is a need for a system and method that allows the seismic cable manufacturer to correctly align the sensors within the seismic cable during the manufacturing process while any inherent torque in the seismic cable is removed.